Method for abandoning an underground storage tank

ABSTRACT

A method of abandoning an underground storage tank including the steps of: removing residuals, if any, from the tank; washing the tank with a detergent solution; removing the washing solution; and filling the tank with a mixture which includes water, sand, a binding agent and a material for enhancing the flowability of the mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to methods and procedures for abandoning underground storage tanks, particularly those which contain or have contained hydrocarbon substances or any other substances of an environmentally hazardous nature.

2. Description of the Prior Art

There are thousands and probably millions of underground storage tanks in which gasoline, fuel oil or other hydrocarbons have been stored. The Environmental Protection Agency indicates that it is aware of at least 1.8 million underground fuel tanks in the United States of which 110,000 leak. Even though large numbers of these tanks may have been abandoned, many of them leak or have leaked residual substances into the surrounding soil. Others may not have yet leaked but will do so in the future. All of such underground storage tanks are potential pollution problems, polluting the surrounding soil and possible water sources. Federal and state agencies are now requiring that these tanks be removed or cleaned up and abandoned in some environmentally safe method.

Abandoned underground storage tanks also create many other related problems. Due to the potential liability from environmental pollution, the owners of property on which underground storage tanks may be situated are frequently prevented from transfer or sale of the property for several reasons. First and foremost potential buyers do not want to assume the risk associated with abandoned underground storage tanks. Secondly, banks and other lending institutions may not be willing to loan money to the owner or purchaser of businesses, such as service stations, garages, etc., where underground storage tanks are involved. Some court decisions have held lenders liable for cleanup costs if they had foreclosed on contaminated property or had merely advised businesses that owned contaminated land. Lending institutions are thus reluctant to expose themselves to such potentially costly problems.

Because of the risks associated with underground storage tanks, new measures are being taken in new installations for detecting and preventing leaking underground storage. In some cases tanks are being placed above ground. However, this does not solve the problem of leaking tanks already in place.

The abandonment of subterranean cavities, such as abandoned mines, has long been of concern. U.S. Pat. No. 1,404,112 discloses a method of filling a subterranean cavity in which a slurry of soil and water is pumped into the cavity behind a temporary wall. The water is allowed to leak through the temporary wall, the soil remaining in position within the cavity. U.S. Pat. No. 3,892,442 discloses a more recent mining approach to this problem in which a foam material is dispensed in a mining shaft to fill the void created by removal of the mined product. Materials mentioned are polystyrene, polyurethane, foam cement, foam plastic, etc.. Appropriate additives make the foam fire retardant.

The most widely utilized and accepted method of abandoning underground storage tanks at the present time is removal of the tanks. However, this is a very expensive, time consuming and business interrupting procedure. If any residual fluids are in the tank, they must be pumped out and disposed of in an acceptable manner. Heavy equipment must be brought in to remove soil surrounding the tank and the tank itself. If the surrounding soil is contaminated, the contaminated soil must be removed, temporariliy contained and eventually disposed of in a manner acceptable to environmental pollution agencies. While waiting for disposal, the contaminated soil, due to vaporization of the contaminating substances therein, may pose health problems to the surrounding area. Then the cavity resulting from removal of the tank and/or the surrounding soil must be filled with uncontaminated soil. Such an operation may require days or even weeks and may be very destructive to other improvements of the subject property such as driveways, sidewalks, etc.. It is most likely that such operations will also disrupt or totally suspend business operations of the business on whose property the tanks are disposed. It can be seen that this creates major problems.

Of course, the costly and disruptive procedure of removing an abandoned underground storage tank could be avoided if an acceptable method were found to abandon these tanks in place. One such method is proposed in U.S. Pat. No. 4,693,284 in which an abandoned underground storage tank is filled with a rigid polyurethane foam. The method revealed therein suggests removing any residual substances and evacuation and/or evaporation of vapors therefrom by forced air ventilation. It is suggested that prior to filling the tank with foam, minor amounts of solid absorbant materials be placed in the tank to absorb condensation, water, mud, or sludge that might be left in the tank. It is not known whether this procedure has been approved by the various environmental pollution agencies. It is suggested in the patent that it might become necessary in the future to remove the tank as a result of regulatory requirements and that if the tank is removed it would be easier to handle filled with foam than not. Even if this procedure is approved and the tank never removed from the ground, a large mass of plastic foam is left in place, rendering a certain area of the property less usable for things such as structural support, water drainage, certain types of landscaping, etc..

As can be understood, the abandonment of underground storage tanks, particularly those in which hydrocarbon materials or other hazardous substances have been stored, has become a major problem. Efforts continue to be made to solve this problem in a safe, cost effective and environmentally accepted manner. Further improvements are needed.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a new method of abandoning an underground storage tank which includes the steps of: removing residual substances, if any, from the tank; washing the tank with a detergent solution; removing the washing solution; and filling the tank with a mixture which includes water, sand, a binding agent and a material for enhancing the flowability of the mixture. If there is any piping connected to the tank, it may be filled with expanding plastic foam prior to filling of the tank with the mixture. The method also may include steps which accelerate decomposition of the tank and which may rehabilitate surrounding contaminated soil by bioremediation.

Thus, the present invention provides a method of abandoning an underground storage tank which most importantly, should be acceptable to regulatory agencies. It provides a method by which an underground storage tank may be abandoned in place, eliminating the costly and business interruptive methods which require removal of the tank. When the method is completed, the tank is filled with materials which provide structural support and should not interfere with future use of the land as might be the case with other methods. In fact, in some embodiments of the invention, the surrounding tank may eventually be totally decomposed.

All operations of the method of the present invention may be conducted through the fill connections of an underground storage tank, avoiding potentially hazardous operations and major construction activity such as removal and installation of drives and parking areas. In most cases, the procedure can be conducted in a short period of time and at any time of the day, resulting in minimum interruption of business activities at the site.

Since requirements of environmental pollution agencies are met and since the procedure is effective and much less costly than other procedures, many of the problems associated with use and/or sale of the property are eliminated. This method should greatly reduce lending institutions reluctance to lend money to the owners or users of property on which underground storage tanks are located. Many other objects and advantages of the invention will be seen from the description which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

Prior to taking active steps to abandon an underground tank, the soil surrounding the tank should be tested for contamination by former tank contents, particularly governmental regulated substances. The size of the tank and the size and length of piping connected thereto should be determined. If there are any residual substances in the tank, the nature of these substances should be determined, if possible, and they should be removed by a vacuum truck, or the like, for proper disposal.

If previous testing reveals that the surrounding soil is contaminated by a regulated substance, the next step may be to introduce hydrocarbon devouring microbes into the tank and to allow bioremediation to take place in the tank and in the surrounding soil for several days. Bioremediation breaks the hydrocarbons down into acceptable products of fatty acids and carbon dioxide. After sufficient bioremediation has taken place, any substances remaining in the tank can be left in place or removed for proper disposal.

There may be some reluctance by regulatory authorities to permit the introduction of hydrocarbon devouring microbes into the soil surrounding a tank. For this reason, it may be more acceptable to introduce certain enzymes into the soil which promote the growth of the natural hydrocarbon devouring microbes which already exist in the soil. In addition, hydrogen peroxide or some other oxygen source may be introduced with the enzymes to enhance the action thereof.

Next the tank is preferably washed with a solution of water and a surface tension reducing surfacant (detergent). This can be done by placing spray nozzles in the tank through the fill connection of the tank. After the tank is sufficiently washed, the washing solution and materials washed thereby are removed for proper disposal.

At this point, any piping connected to the tank should be filled. Before filling the piping, any residual materials therein should have been removed. The piping is preferably filled with an expanding plastic foam or the like. A suitable mixture for producing expanding urethane foam compatible with tank residue would be an isocyanate compound and polyhydric alcohol. These components are simply mixed together in a suitable ratio, e.g. one to one by volume, to produce expanding urethane foam. Since the size and length of the piping has been previously determined, the amount of foam materials necessary for filling the volume of the piping is introduced in the piping at some distance from the tank and forced toward the tank by the expanding foam. It usually takes anywhere from thirty seconds to two minutes for the foam to expand and fill the piping with an inert material. This material may typically have a ten or twenty to one expansion rate. The ends of the piping are then capped.

Then the tank is filled with a mixture which includes water, sand, a binding agent and a material for enhancing the flowability of the mixture. A typical composition for the mixture, in percentage by volume, would be:

    ______________________________________                                         Material        Volume by %                                                    ______________________________________                                         Sand            55.0%                                                          Water           22.0%                                                          Fly Ash         18.0%                                                          Sodium Silicate 3.7%                                                           Triacetin       0.3%                                                           Metal Salts     0.7%                                                           ______________________________________                                    

Of course the percentages may vary. For example, the liquids (water, sodium solicate, triacetin, metal salts) may vary between 23% and 31% and the solids (sand, fly ash) may vary between 69% and 77%.

Fly ash is included in the mixture to enchance the flowability of the mixture as it is being introduced into the tank. Other materials may be used for this purpose. Other materials suitable for this purpose are cement fumed silica, adopulgite clay, dry flow starch, kiln dust, etc..

Sodium silicate is the material which binds the materials of the mixture together as it sets up. Triacetin is a catalyst for the binder. Another catalyst which would decrease the reaction time is muriatic acid. Other catalysts are citric acid and sodium flow silicate (sodium silica fluoride).

Metal salts may be included in the mixture to provide an electrolyte to enhance corrosion and eventual decomposition of the metal tank. Thus, the metal salts used should be those metals which are more noble than ferrous.

Of course, the greatest advantage of the procedure of the present invention is the abandonment of an underground storage tank without having to remove the tank from the ground. The complete procedure may be completed within a few hours and may be done after normal business hours. Thus, interruption of business and business facilities is reduced to a minimum. The abandoned tank is left filled with a solid, basically inert material which is capable of supporting structural loads much as the surrounding soil. This procedure should alleviate many of the problems associated with the sale and lending of money on projects associated with the property.

The method of the present invention has been described in its preferred form. However, it should be understood that this method and the steps thereof can be varied considerably without departing from the spirit of the invention. Accordingly, it is intended that the scope of the invention be limited only by the claims which follow. 

We claim:
 1. A method of abandoning an underground storage tank in which hydrocarbon substances have been stored comprising the steps of:a. removing residual substances, if any, from said tank; b. washing said tank with a detergent solution; c. removing said detergent solution; and d. completely filling said tank with a mixture which includes water, sand, a binding agent and a material for enhancing the flowability of said mixture wherein said mixture forms a solid composition with a structural strength similar to that of the surrounding soil.
 2. The method of claim 1 in which said storage tank is made of a ferrous material, said mixture including metal salts more noble than said ferrous material to enhance the corrosion of said storage tank.
 3. The method of claim 1 in which any piping connected to said tank is filled with an inert material prior to filling said tank with said mixture.
 4. The method of claim 3 in which said inert material is produced by filling said piping with expanding plastic foam.
 5. The method of claim 4 in which said expanding plastic foam is created by mixing an isocyanate compound with polyhydric alcohol.
 6. The method of claim 1 in which said mixture includes a catalyst for said binding agent.
 7. The method of claim 6 in which said catalyst is selected from the group consisting of: triacetin, muriatic acid, citric acid and sodium silica fluoride.
 8. The method of claim 1 in which said material for enhancing flowability is selected from the group consisting of: fly ash, fumed silica, attapulgite clay, dry flow starch and kiln dust.
 9. The method of claim 1 in which said mixture includes by volume 23% to 31% liquids and 69% to 77% solids.
 10. The method of claim 9 in which said mixture includes by volume approximately 55% sand.
 11. The method of claim 9 in which said mixture includes by volume approximately 22% water.
 12. The method of claim 9 in which said material for enhancing flowability makes up by volume approximately 18% of said mixture.
 13. The method of claim 1 in which said mixture includes by volume the approximate percentages of the following materials: 55.0% sand, 22.0% water, 18.0% fly ash, 3.7% sodium silicate, 0.3% triacetin.
 14. The method of claim 13 in which said mixture also includes by volume approximately 0.7% metal salts.
 15. The method of claim 1 including the step of introducing hydrocarbon devouring microbes into said tank for bioremediation of any residual hydrocarbon substance in said tank prior to filling of said tank with said mixture.
 16. The method of claim 1 in which the soil surrounding said tank is tested for the presence of hydrocarbon contaminating substances, and if any are detected, introducing enzymes into the soil for promoting the growth of natural hydrocarbon devouring microbes therein.
 17. The method of claim 16 in which a source of oxygen is introduced into said soil for enhancing the growth promotion of microbes by said enzymes.
 18. The method of claim 1 in which the binding agent is sodium silicate. 